It is important to know the nature and character of soil at selected locations in the vadose or unsaturated zone to determine if it may contain toxic materials or may be contaminated in some way which would make it suitable or unsuitable for specific purposes.
Water, which is contained within the soil, is representative of the soil in many respects, and the art has sought effective ways to collect the soil water at selected locations so that the water obtained may be analyzed, its contents identified, and their concentrations determined.
A method used in early attempts to recover soil water involved taking a sample of the soil and putting it under pressure to mechanically squeeze out the water. Devices were later developed called lysimeters which contain a sampling chamber and which use a vacuum to draw moisture from the soil through a filter into the sampling chamber. These devices have their limitations and they introduce inaccuracies in the collection of the water so that the soil moisture recovered has not been truly representative of the soil moisture under test. In an effort to reduce these inaccuracies there were attempts to use polyethylene porous tubes in the lysimeter. Then the focus of the art was turned to the use of porous ceramic filters, and attempts were made to preclean and treat these ceramic filters by leaching the porous ceramic material with hydrochloric acid followed by rinsing with deionized water. The porous ceramic filters were found to absorb or to interfere with the chemical constituents NO.sub.4, NO.sub.3, NO.sub.2, PO.sub.4, P and S especially when the sorptive capacity of the soil is less than that of the ceramic material. The porous ceramic filters were found to attenuate concentrations of Ni, Cu, Pb, Z.sub.n, Fe and Mg, and were found to be unsuitable for fecal coliform analysis.
When using porous ceramic materials in the lysimeter, the porous ceramic material usually has been attached to a polyvinylchloride (PVC) tubing by use of an adhesive, but it has been found that both the PVC tubing and the adhesive used to secure it contribute to inaccuracies of the collected sample. Further, the ceramics are extremely fragile and it is difficult to assemble the ceramic filters in lysimeters, to install the lysimeters in a selected location and to remove the soil water collected without fracturing or breaking the ceramic materials.
An object of the present invention is to discover how to make an improved filter material which is structurally strong and chemically inert which can be used in a lysimeter through which moisture from the soil may be passed and then recovered from the lysimeter for testing, without disturbing the characteristics and contents of the soil moisture and without contributing elements to or subtracting elements from the soil water due to the material itself, and which can be used for collecting soil water which is truly representative of the water contained in the soil being tested.
Although there have been many known substances which may be considered to be inert, to my knowledge none of these have characteristics of moisture transmission which would make them suitable for use in soil water recovery.
There is a type of plastic resins which are basically monomers containing one or more atoms of fluorine or copolymers of such monomers with other monomers, the fluorine-containing monomers being the greater part of the mass. These plastics are called fluoroplastics and have been used as coatings, linings, and as components of pumps, fittings, process vessels etc. Fluoroplastics include polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), ethylene-chlorofluorethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), ethylenetetrafluoroethylene copolymer (ETFE), perfluoroalkosy resin (PFA), and fluorinated ethylene-propylene copolymer (FEP). As now known and used, these resins are neither rigid nor porous. They present a tight barrier to transmission of either liquids or gases. They would clearly be unsuitable for any filtering function.
I have discovered that it is possible to use these fluoroplastic resins as raw materials in a process for preparing synthetic resins which are rigid and porous, which can be molded and machined as filter units in a lysimeter and that a lysimeter which employs such special filter units is much more effective in the collection of soil water which is truly representative in content and character of the moisture contained in the soil at the location selected for test.
Preparation of the Special Resin
As a first step the fluoroplastic raw material such as PTFE is blended with a sacrificial agent. The sacrificial agent is a raw material used in the process but which is totally removed during the course of the process. I prefer to use stearic acid, but other fatty acids or chemicals having vaporization points of the order of 350.degree. to 450.degree. F. under the conditions of the process, may be used.
The fluoroplastic should be in powdered form, in particles which will pass a screen of 6 to 100 mesh, preferably 10 to 20 mesh, and the sacrificial agent should also be in powdered form, in particles which will pass a screen of 15 to 325 mesh, preferably a screen of 100 to 200 mesh.
The fluoroplastic resin and the sacrificial agent in a proportion of about 5% to 99% fluoroplastic and about 95% to 1% sacrificial agent, preferably about 80 to 90% resin and 10% to 20% sacrificial agent. Using these preferred proportions it may be expected to produce pores with diameters of the order of about 70 microns. In general lower percentages of sacrificial agent produce a product having a lower degree of porosity and and higher percentages of sacrificial agent produce products having a higher degree of porosity. Using the above as a guide, pore sizes of from about 0.004 to about 300 microns may be obtained and used in my process.
The fluoroplastic and the sacrificial agent may be placed together in a rotary screen which, preferably is made of polyvinylchloride, and the mixture thoroughly blended. It is important to avoid lumps in either the fluoroplastic or the sacrificial agent as this would produce faulty areas without pores or passages. The blend thus produced is ready for molding.
Molding of the blend of resin and sacrificial agent
If it is desired to make the filter in tubular form a suitable size cylindrical mold having a central core, suitably made of polyvinylchloride, may be selected. The mixture is fed into the mold and pressure applied on the mixture. The pressure initially applied should preferably not exceed 3000 p.s.i. but the pressure may be increased to the order of about 5000 p.s.i. and may be held for a time (suitably about 30 seconds to 1 minute) before being released. The pressed and formed material may be removed from the mold. Some care should be taken in doing this because at this stage the material is somewhat fragile.
Removal of the sacrificial agent
After removal of the product from the mold it may be placed in an oven having a relatively low temperature (about 350.degree. F.-450.degree.F.) and held for a time sufficient to completely volatize the sacrificial agent. For a tubular piece having an O.D. of about 2 inches and about 6" long a time of 2 to 6 hours should be satisfactory. In general, larger pieces take longer times to effect complete vaporization while smaller pieces require less time. When the sacrificial agent is completely volatilized the piece will be seen to have a blackened appearance.
The Sintering Treatment
After the sacrificial agent is completely volatilized from the resin material it may be placed in a relatively high temperature oven having a temperature of about 600.degree. F. to 750.degree. F., preferably about 690.degree. F. for a time (a minimum of about 48 hours) or until the material appears luminescent and semi-transparent, its former blackened appearance having completely disappeared. After this high temperature treatment is complete the material may be allowed to cool (preferably in the oven until it comes to about 400.degree. F.). I find that even if the material is left in the high temperature oven for a longer time than above indicated the material does not change in character and remains usable in a lysimeter in the same way. When the material comes down to substantially ambient temperature it may be machined to prepare it for assembly within a lysimeter. For example, threads (preferably square threads) may be formed on its ends for making attachments to other parts of the lysimeter.